1. Field of the Invention
The present invention relates to a thin-film coating apparatus, and more particularly to a thin-film coating apparatus of a type in which a film-forming coating solution is applied in a dropwise manner onto the surface of a material to be treated, and is spread by rotating the material to provide a thin film coating thereon.
2. Description of Relevant Art
In a known type of thin-film coating apparatus, a coating solution is dropped onto the surface of a material to be treated such as a metal plate, glass plate, ceramic plate, printed circuit board, semiconductor wafer or the like, and the material is then rotated by employing a spinner. An exemplary apparatus for providing a coating of a photoresist or diffusion source for semiconductors on a surface of a semiconductive wafer is illustrated in FIG. 8 of the accompanying drawings.
With reference to FIG. 8, the coating apparatus comprises a casing 101, a spinner shaft 103 rotatably supported on a bottom wall 102 of the casing 101, a spinner head 104 provided on the spinner shaft 103 and a nozzle 105 disposed above the spinner shaft 103 and adapted to drop a coating solution. A through-hole is formed through both the spinner shaft 103 and the spinner head 104. By connecting the through-hole for communication with a vacuum pump, a wafer 106 mounted on the spinner head 104 can be closely attached to the spinner head 104, so that the spinner head 104 and wafer 106 rotate as an integral unit.
When the coating solution is applied in a dropwise manner from the nozzle 105 onto the wafer 106 and the wafer 106 is rotated at a high speed, a part of the thus applied coating solution is uniformly radially spread on the surface of the wafer 106 by virtue of centrifugal forces. A major portion of the thus applied coating solution does not take part in the formation of a coating film, but rather flies off from the surface of the wafer 106. It then hits the inner peripheral wall of the casing 101, flows down along the wall surface, and is finally collected through a drainage opening 107.
Such conventional apparatus is generally provided with a device which evacuates the casing to prevent the flying particles of coating solution from sticking to the surface of a wafer being treated. Therefore, the interior of the apparatus is kept at a reduced pressure. On the other hand, convection occurs in the apparatus because the spinner head and wafer are rotated at a high speed. Accordingly, the atmosphere within the casing promotes volatization of the solvent, resulting in accelerated evaporation of the solvent from the coating solution.
Due to such accelerated evaporation, the solvent evaporates easily from the coating solution which is flowing down along the wall surface of the casing, thereby causing its solute to be deposited and become stuck to the wall surface of the casing or the like. If the deposit of the solute is accumulated there, pieces of the solute are blown upwardly so as to stick to a wafer being treated, with the result that the coated film develops pin holes, includes projections made of pieces of the solute, and/or becomes uneven, thereby leading to a lowered product yield.
Heretofore, in order to overcome the aforesaid disadvantages, the wall surface has been wiped with sponge, cloth, filter paper or the like, which may optionally be impregnated with a solvent; the wall portion of the casing has been disassembled and washed; or a lining such as an aluminum foil or a film sheet has been applied on the wall surface of the casing.
However, the above solutions have been totally dependent on manual work, and impede any speed-up of the coating step. In addition, the manual work prevents carrying out of the coating step and its preceding and subsequent steps as a series of continuous operations, thereby hampering the automation of the overall coating operation. Thus, such conventional solutions are disadvantageous from the standpoint of production efficiency.
The present invention effectively overcomes the above-described disadvantages attendant the conventional techniques.